1. Field of the Invention
The invention relates to a deflecting system for deflecting a charged particle beam whereby the deflecting system comprises at least two deflectors. The invention further relates to a charged particle beam device comprising a deflecting system to separate the charged particle beam from secondary charged particles.
2. Description of the Related Art
Charged particle beam devices like electron beam microscopes or electron beam pattern generators use charged particle beams to probe or structure a specimen with high spatial resolution. For the high spatial resolution, the charged particle beam has to be directed and focused with high precision to the position on the specimen that is to be probed or structured. The forming of the charged particle beam is usually performed by electric or magnetic lenses and apertures. For directing a charged particle beam, deflectors are used which use electric or magnetic dipole fields to control deflections of the charged particle beam from a first direction to a second direction.
Unfortunately, deflections of charged particle beams are known to introduce an astigmatism to a charged particle beam. Astigmatism of a beam optical device causes a focused charged particle beam with a previously circular beam cross section to become deformed to an ellipsoidal shaped cross section. The ellipsoidal shaped cross section is the result of a projection where the focusing length in a first plane along the beam axis (sagittal plane) different from the focusing length in a second plane along the beam axis (meridional plane) which is vertical to the first plane. The size of an astigmatism is usually expressed by the difference between the two focusing lengths of the charged particle beam. Alternatively, the size of an astigmatism can be expressed by the diameter of the beam spot of the charged particle beam at a position where the focus is circular (circle of least confusion), i.e. where the focus size of the first plane equals the focus size of the second plane. More details about how deflectors introduce astigmatism are described, for example, in P. W. Hawkes and E. Kasper: “Principles of Electron Optics” Vol. 1, Chapter 32, pp. 483–521 and Vol. 2, Chapter 40, pp. 823–854, Academic Press 1989.
Astigmatism is problematic for a high spatial resolution operation, since the probing area of an astigmatic deformed beam is larger than the probing area of the previously circular cross section of the beam. This worsens the achievable spatial resolution. Even worse, astigmatism may accumulate with each deflection which for a beam optical design limits the number of deflections as well as the maximum deflection angles to meet given spatial resolution requirements.
Astigmatism, therefore, is a severe problem for charged particle beam devices where the primary charged particle beam is experiencing several deflections in a row, such as, for example, in a scanning electron microscope (SEM) where magnetic deflectors are used as separators to separate the primary charged particle beam from the beam of secondary charged particles generated by the primary charged particle beam on the specimen. An example for such a deflecting system can be found in the international patent application W099/26272. The separation of the primary charged particle beam particles from the secondary charged particles is based on the fact that they enter the magnetic deflector from opposite directions. Since the Lorenz-Force of the magnetic field depends on the direction of the incoming charged particles, both beams are deflected in opposite directions. This way, it is possible to detect the secondary charged particles with a detector which is not in the way of the primary charged particle beam.
However, after separating the primary charged particle beam from the secondary charged particles, it is usually preferred that the primary charged particle beam is redirected back onto the optical axis of the beam optical system. Therefore, at least two, and, in most cases, at least three or four deflectors are used.
The advantage of a four stage deflection system that the deflections necessary to return the primary charged particle beam back onto the optical beam axis can be distributed over four deflections having the same deflection angles. Having the same deflection angles allows for a use of the same deflection coils and the same coil current which simplifies the beam optical design considerably. However, the more deflecting stages the deflection system has, the larger the astigmatism accumulated over the various deflection stages.
Astigmatism becomes even more a problem when a charged particle beam device comprises a two-dimensional deflecting system consisting of two deflecting systems which deflect the charged particle beam within two different deflection planes, e.g. within the orthogonal X-plane and Y-planes. With a two-dimensional deflecting system, the primary charged particle beam and the secondary charged particles can be deflected within the X-plane, within the Y-plane and within any plane in between depending on the relative deflecting forces of the two deflecting systems. A two-dimensional deflecting system useful because it makes it possible to switch between different deflection planes to ensure that the secondary charged particles, for each scanning point, are directed to different directions and, possibly, to different detectors. This way, a specimen can be analyzed in parallel with different detectors which can greatly improve the diagnosis capabilities of a charged particle beam device.
However, a two-dimensional deflecting system comprises twice as many deflectors as a comparable one-dimensional deflecting system. Therefore, the astigmatism of a two-dimensional deflecting system, depending on the relative deflecting forces of the two deflecting systems, is significantly increased.